Transmission clustering drives the onward spread of the HIV epidemic among men who have sex with men in Quebec

Abstract

Phylodynamic analysis and epidemiologic data identified 3 patterns of spread of primary human immunodeficiency virus type 1 infection (PHI) among men who have sex with men (2001-2009): 420 unique PHIs, 102 small clusters (2-4 PHIs per cluster, n = 280), and 46 large clusters (5-31 PHIs per cluster, n = 450). Large clusters disproportionately increased from 25.2% of PHIs in 2005 to 39.1% in 2009 (χ(2) = 33.9, P < .001). Scalar expansion of large clusters over 11 months (interquartile range, 3.5-25.5 months) correlated with cluster membership size (r(2) = 0.174, F = 4.424, P = .047). PHI cohort data revealed variations in social networks and risk behaviors among the 3 groups, suggesting the need for tailored prevention measures.

Figure 1.

A representative region of the maximum likelihood phylogenetic tree (n = 214 of 1150). Sequences in small (n = 56) and large (n = 99) transmission clusters are denoted by red and blue triangles, respectively. Unique transmissions (n = 59) are denoted in black with any observed clustering attributable to repeat patient sampling. Sequences can be accessed via GenBank (accession numbers JF957375–JF957589).

Figure 2.

Episodic intervals of large transmission clusters among men who have sex with men (7–31 primary human immunodeficiency virus infections [PHI]). A, Box and Tukey whisker plots depict median, mean (+ sign), and episodic range (1.5 times IQR) of the 23 individual clusters occurring from 2001 to 2009. PHI beyond Tukey whiskers are shown as single points. B, The interrelationship between the size of the 23 clusters and median cluster duration (months). C, Frequency histogram of distance in months of individual PHI (n = 311) from the median dates of their respective clusters.